1. Field of the Invention
The invention pertains to a voltage transformer with a measuring element disposed in the electric field between an internal conductor and an external conductor of a metal-encapsulated, preferably gas-insulated switchgear and which is made of a clearly transparent crystal material which, under the influence of the electric field, influences polarized light that is radiated through on account of the so-called Pockels effect.
2. Description of the Related Art
Voltage transformers in which a crystal exhibiting the Pockels effect (an electro-optical effect) is utilized have been disclosed, for example, in the publication DE-B Schwab, Hoch-spannungstechnik [High-voltage technology], 2d Ed., Springer Verlag, page 176 et seq., in particular 177-179, 181-184. In contrast to so-called conventional voltage transformers, voltage transformers which utilize the Pockels effect can measure high-frequency field or voltage changes. In that context, potential isolation is provided; instances of ferroresonance do not occur and the system is insensitive to electromagnetic interference. A voltage transformer in which the Pockels effect is utilized has become known from European patent publication EP 0 410 234 B1, for example. The crystal used in that case is a quartz crystal and the light which is produced by a light source and polarized by a polarizer radiates through the quartz crystal in a direction running perpendicularly to the field lines of the electric field. On the output side, the light beam, which is influenced by the crystal, is split into two light beams with a beam splitter. The beams each pass through an analyser and are forwarded to a receiver through optical fibers.
Voltage transformers in which the Pockels effect or a similar effect is utilized have also been disclosed in U.S. Pat. No. 3,363,174. In that case, light radiates through a rod made of a crystal. The light beam radiates through the crystal in the direction of the field lines or parallel thereto, and a photocell is provided, which captures the light modulated by the rod-shaped crystal and processes it further. The system is designed as an outdoor voltage transformer and it is not suitable for SF.sub.6 -gas-insulated, metal-encapsulated switchgear.
A voltmeter of the type mentioned above has also been disclosed in the European patent publication EP 0 083 196 B1. In that case, use is made not only of crystals of KDP, ADP, LiNbO.sub.3 and LiTaO.sub.3, but also of crystals which are referred to as BGO (with a composition of Bi.sub.12 GeO.sub.20). The crystal has two electrodes, which are transparent. The light beam runs in the direction of the electric field. The light beam is first of all directed through a polarizer, a quarter-wave plate, the crystal and through an analyser, and the light influenced by the analyser is processed further. The document describes the voltage transformer only in principle.
A further literature reference is the SUMITOMO ELECTRIC company journal, which describes a measuring device in which the crystal is arranged in a core which is placed around an electrical conductor. The crystal material is BSO.